1. Field of the Invention
This invention relates to satellite communication systems and methods, and more particularly to the broadcast of signals on a local area basis with some of the signal frequency bands repeated for different areas.
2. Description of the Related Art
Cable television systems have been used to provide local television service, with the programming content differing from one service area to the next, in addition to nation-wide program distribution in which all areas receive the same national programming. While satellite broadcasting has also been successfully used for nation-wide broadcasts, local area service has proven more difficult to achieve because of interference between signals intended for different service areas that have different program content. In the past, satellite broadcasts have been limited to a generally uniform large regional coverage, such as the entire United States, without the inclusion of local service broadcasts.
xe2x80x9cSpotxe2x80x9d broadcast beams, which are smaller than regional beams, have been used previously for non-television satellite broadcasting, such as telephone applications. Two types of spot beam broadcasts have been employed. In one, illustrated in FIG. 1, a desired region 10 such as a country is covered by a uniform grid of evenly spaced spot beams 12 having equal sizes and output power levels. To assure complete area coverage, adjacent beam spots are overlapped. Different and non-overlapping frequency bands are assigned to the signals within each pair of adjacent beams to prevent cross-beam signal interference. In the simplified illustration of FIG. 1, four different frequency bands are employed (designated #1, #2, #3 and #4), with each beam separated from the next closest beam with the same frequency band by at least one other beam having a different frequency band.
The uniform spot beams 12 provide a complete coverage of the desired larger regional area 10, without significant interference between beams. However, a distinct disadvantage of this approach is that the satellite""s resources are evenly divided among target areas of key importance, such as high density population centers, and target areas of much lesser importance such as mountainous and other less developed areas. This can result in either an overly complex satellite system, or a system that does not provide adequate capacity to the most important target areas.
A second approach has been to broadcast different beams having signals within a common frequency band to separate target areas that are spaced far enough apart from each other to avoid significant cross-beam interference, thus allowing for a higher signal capacity to those areas that are covered. The different beams can be broadcast with different output powers, thus providing the greatest capacity for the most important target areas. However, the requirement that the beam target areas be spaced well apart from each other can result,in an inadequate overall coverage area, and the broadcast signals are limited to only a single frequency band.
Other U.S. patents to Acampora, U.S. Pat. No. 4,315,262, and to Assai, U.S. Pat. No. 4,868,886, describe spot beam satellite arrangements for use with point-to-point communication such as telephony. Acampora describes scanning spot beams over different parallel strip zones having similar traffic demands. Assai describes a system that can provide either a global beam or simultaneous global and spot beams. Neither one appears to be applicable to a high speed digital system which is required for digital television transmission to multiple population centers of various size by using nonuniform sized spot beams.
The present invention provides a new and improved, highly efficient system and method for satellite broadcast of local television and other types of service, either independently or together with larger regional broadcasts. Both bandwidth efficiency and communications link performance are significantly improved, with interference levels reduced for the most important service areas. It allows for a higher overall system throughput to a given geographic region, and is economically viable because of its increased capacity and accommodation of marketplace realities in those areas.
These advantages are achieved by broadcasting multiple spot beams from a spacecraft, such as a satellite, to different target areas in a non-uniform beam pattern, and providing at least some of the beams with different respective signal frequency bands. However, at least some of the beams have a common frequency band, and such beams are directed to non-overlapping target area locations to avoid excessive interference. Priorities are established among different target areas by assigning different sizes and powers to different beams, with the higher power beams accommodating larger signal capacity and also resulting in a lower interference level from other beams. The priorities among different target areas can also be set by the selection of antenna reflector sizes to produce different roll-off characteristics for different beams, and by varying the illumination tapers of different antenna feed horns to establish different peak-to-edge power differentials for different beams.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.